Fix SFTP session hang after authentication

[VulcanoSoftware]

Fixed a bug in the SFTP server implementation where the session would hang after successful authentication. The issue was caused by an incorrect implementation of the sftp_factory in asyncssh, which was returning an already-instantiated DCFSSFTPHandler instead of a callable (as expected by the library).

The fix involved:

1. Modifying create_sftp_server in dcfs/app/sftp/__init__.py to make sftp_factory return a lambda that takes the SSHServerChannel and creates the handler.
2. Updating DCFSSFTPHandler.__init__ in dcfs/app/sftp/handler.py to explicitly accept the channel and pass it to the asyncssh.SFTPServer base class.
3. Adding a unit test tests/test_sftp_factory.py that mocks the asyncssh interaction to ensure the factory and handler are correctly initialized.

Verified the fix with a reproduction script and confirmed that existing tests still pass.

---

PR created automatically by Jules for task 10688922256061701937 started by @VulcanoSoftware

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.